Some of the challenges in processing whole or partial crudes can be related to the balance between the cost of upgrading certain types of fractions and the resources available on-site at a refinery. For example, many types of upgrading processes involve consumption of hydrogen. Due to the limited number of sources of hydrogen available in a refinery, it is preferable to reduce or minimize the amount of hydrogen that is used for upgrading of low value streams to low value products. Processes such as fluid catalytic cracking (FCC) can partially mitigate this problem by providing a method for upgrading lower value heavy fractions to naphtha fractions suitable for use in a gasoline pool. However, FCC processes provide only a partial solution, as such processes also typically generate substantial amounts of heavier fractions that may not even have value as fuel oil without further processing.
One reason that a lower value fraction may appear to require processing in the presence of additional hydrogen is due to the sulfur content of a fraction. Hydroprocessing for sulfur removal can be effective for removal of substantially all of the sulfur in a given petroleum fraction. Unfortunately, the hydroprocessing catalysts and/or conditions that lead to sulfur removal can also typically lead to substantial saturation of aromatic rings within a fraction. Such aromatic saturation can increase the amount of hydrogen consumed during hydroprocessing by as much as an order of magnitude or more. Because of the limited nature of hydrogen availability in some refineries, the excess hydrogen required for hydroprocessing of highly aromatic fractions can make upgrading of such fractions undesirable.
U.S. Pat. No. 3,788,978 describes methods for desulfurization of heavy hydrocarbon fractions using molten sodium. The methods are described as being suitable for performing substantial conversion of the heavy hydrocarbon feed, including conversion of substantially all asphaltenes within a heavy hydrocarbon feed, as well as removing 90 wt % of the sulfur in the heavy hydrocarbon feeds. The described levels of feed conversion and desulfurization are enabled by performing the molten sodium desulfurization at a temperature of 750° F. (399° C.) or more. The molten sodium desulfurization may be performed after an initial hydroprocessing step for removal of sulfur.